In general, magnesium having high energy density per unit mass and volume is a material with good prospects as an anode material for a battery. Particularly, since magnesium has a lot of resources and is handled with ease, a magnesium secondary battery has excellent safety and price competitiveness, and thus has been given many attentions as a middle- or large-size battery for electric power energy storage or for electric vehicles. Also, it is expected that such a battery realizes significant market expansion.
The theoretical energy density of a magnesium secondary battery is relatively high, while the theoretical energy density of a lithium secondary battery is the highest. However, realization of the characteristics of magnesium as a secondary battery has been disclosed first by T. Gregory et. al, in 1990. Any studies about magnesium batteries have not been conducted for about 10 years since 1990. In the 2000s, BIU Group has developed Chevrel-phase cathode active materials ensuring reversibility. Due to this, magnesium batteries have been given many attentions as a substitute capable of solving the problems of safety and cost related with lithium secondary batteries. However, the energy density of magnesium secondary batteries that have been developed to date is merely about ½ or less of that of lithium secondary batteries. Under these circumstances, there is an imminent need for a novel cathode active material, electrolyte solution, collector, or the like.
Particularly, there are many problems to be solved, including precipitation at an anode, reversible dissolution, establishing a cathode process, improving diffusion rate of Mg2+ ions in a solid phase, or the like. The key solving those problems consists in developing a novel electrolyte material system applicable to both a cathode and an anode.
Recent studies are mainly focused on cathode active materials and electrolyte solutions. Referring to cathode active materials, metal sulfur compounds, organosulfur compounds, metal oxides, metal silicate compounds, etc. have been studied to increase the reversible capacity per unit weight and to improve the reversibility, but are not successful. In the state of art, Chevrel-phase, Mo6S8 cathode active materials provide a battery with quality amenable to commercialization.
Referring to electrolyte solutions, Grignard solutions showing magnesium anode reversibility has been studied intensively. Recently, it has been reported that magnesium aluminate shows high quality. However, until now, such Grignard electrolyte solutions in which magnesium is precipitated and dissolved reversibly are not applied to a cathode process due to the high reducing capability thereof. On the contrary, it is difficult to carry out precipitation and dissolution of anode magnesium metal reversibly, in an organic solvent-based electrolyte solutions to/from which Mg2+ ions are intercalated/deintercalated at a cathode active material.
According to the related art (U.S. Pat. No. 6,713,213 published on Jun. 21, 2001, Matsushita Electric Industrial Company), a non-aqueous electrolyte magnesium secondary battery is disclosed, and the battery uses a halogen-containing organomagnesium compound represented by the formula of RMgX as non-aqueous electrolyte, and includes a rechargeable cathode, non-aqueous electrolyte and an anode. In addition, Japanese Laid-Open Patent Publication No. 2007-188709 (Sony Company, Jan. 12, 2006) discloses an electrochemical device having a first electrode, a second electrode and an electrolyte, wherein the active material of the second electrode is designed to be oxidized to produce magnesium ions, and the electrolyte is a composition of Grignard reagent RMgX (wherein R is an alkyl or aryl group, and X is fluorine, chlorine or bromine) with an organometallic compound or salt other than magnesium salts.
However, the above-mentioned electrolytes according to the related art show low ion conductivity and provide a battery with a low charge/discharge rate, and thus has a limitation in battery quality. Therefore, it is necessary to improve an electrolyte in order to develop a magnesium secondary battery having a competitive advantage as compared to the secondary batteries according to the related art.